Coloring aluminum alloys



Patented 2 9, 1931 UNITED STATES ALADAR PACZ, 0F GLEVELAN'D, OHIO COLORING ALUMINUM ALLOYS Ito Drawing. Application flied April 23, 1930, Serial No. 446,791, and in Germany August 10, 1929.

The subject of the invention is a process for the production of colored protective lay ers, grey or blackish for example, on alum num alloys, especially those containing s lia con, and/or zinc, and on articles made of such alloys.

it is well known to provide aluminum and aluminum. alloys wit protective coatings, which resist the action of weather and simi- 1o lar corrosive influences, by an oxidizing treatment with alkaline chromate solutions,

um chromate), relatively larger amounts of 25 substances with an alkaline reaction, such as sodium carbonate, caustic soda or the like.

in the form oi the ordinary chromates or as dichromates (for example potassium 1 dichromate) is below about 0.25% (and preterably below 0.2%) by weight, calculated as I potassium dichromates, of the treating solution. The amount of the alkalies or substances with an alkaline reaction, should be 10: 1. Instead of employing the chromates singly-for example as sodium or potassium 1 chromate or dichromate-or single alkalies-e such vas sodium or potassium carbonate, sodi- 50 um or potassium hydroxide, and the like at least 1.5% (preferably about 2% or over) by weight, calculated as, orequivalent to,

' least 5: 1, but is preferably not below 8: 1 to In the process of the present invention, in contradistinction to the known processes,

neither an oxidic nor a metallic coating is formed on the articles treated, but, on the contrary, by virtue of the action of the relatively largeamount of alkali contained in the solutions, aluminum is dissolved out of the metallic surface to a certain depth, thereby exposing and coloring the alloying components, such as silicon, manganese and the like, whilst-according to the experience of the applicantthe very small amount of chromate present serves to distribute the said alloying components exposed by the dissolving out of the aluminumover the residual surface and to fix same thereto. The process when performed in the manner hereinbefore described, and preferably at elevated temperatures, such as near the boiling point of the liquid employed in the treatment, occupies only an exceedingly short time, so that, in most cases, handsomely colored and durable protective layers can be obtained in a few (e. g. 2) minutes, Whereas, with the other, The amount of chromates employed, Whether kn own processes, the time required is far longer and may extend to several hours.

The alloys suitable for treatment comprise all the known aluminum alloys, those containing silicon and zincv in particular; and preferably, for example, the aluminum silicon alloys known under the registered trademarks, Silumin and Alpax; also the al-. loys containing, for example, manganese, antimony, molybdenum, copper, silver, tin, zinc, tungsten and the like. It is possible by suitably varying the alloying components, to influence the shade of the resulting-protective layer as desired, within certain limits. It has also been ascertained that similar effects can also be obtained by the addition of very small quantities of the salts of certain metals,-such solutions. I

In general, it has been found desirable,

as those enumerated aboveto thetreating especially in the treatment of aluminum- 7 alloy castings, to subject these articles to a preliminary mechanical treatment of known kind, such as sandblasting, scrubbing with wire brushes, and the like, which will remove any superficial film of oxide present and facilitate the action of the alkaline solution on the clean metallic surface thereby exposed.

Ewample l I An article composed of an alloy containing about 87% of aluminum and 13% of silicon is first subjected to mechanical treatment with a revolving wire brush, and is then dipped into an aqueous solution,heated to about 90 G.containing 1 grm. of potassium dichromate and 15 grms. of sodium carbonate (Na CO or the equivalent quantity of caustic soda, per litre. The formation of the superficial layer will be complete in about two minutes, whereupon the article is taken out of the bath, well washed with water and dried in the usual manner. protective layer formed thereon is dark grey in color, witha high lustre, and displays high powers of resistance to moisture, neutral saline solutions and the like.

Ewample II An article composed of an alloy containing about 90% of aluminum and 10% of zinc, is first treated mechanically, for example, by sandblasting, and is then dipped mto an aqueous solution, at about 90 C. con- T taining 2 grms. of potassium dichromate and loys which contains a small amount of a soluble dichromate salt, not exceeding -.02% of the weight of the solution in company with at least about 1.5% of alkali carbonate or by. droxide. v

2. A dipping solution as set forth in claim 1 in which the ratio of alkali to chromate is greater than five to one.

In testimony whereof I hereunto afiix my signature.

A] iADAR PACZ.

The 

